In my application Ser. No. 803,796 there is described a lock especially designed for use on railroad switches to reduch acts of vandalism. The losk disclosed therein comprises a housing adapted to receive an end of a shackle in locking engagement. The lock is operated by rotation of a shaft, which is biased to the locking position. Hence to assemble the shackle with the housing, it is necessary to rotate the shaft to the unlocking position with a suitable tool, insert the shackle, and release the tool. This operation has been found difficult to accomplish under service conditions.